What percentage of users have completion issues in your LMS, assuming the course works correctly?

There are several things possibly at play here. Yes, it can be network security issues sometimes blocking the communication between the user and the server sending the completion trigger. The biggest culprit tends to be completion based on quiz results. This is not recommended as this can have issues. Use quizzes as a reinforcement during the module but pass based on slide completion. Also use Complete/Incomplete as the pass status as many LMS's do not recommend a status with pass or fail in them as this has been known ot be problematic.